Extrusion dies form a flowable material, such as a thermoplastic melt, a solvent solution, an emulsion, a radiation curable oligomer and the like, into an unsupported sheet, film, or web. In a common application in the plastics industry, a molten, pressurized stream of a thermoplastic material is moved into an inlet, through an internal distributing cavity, and discharged from an exiting slot to form a ribbon of material which is solidified to continuously form a sheet, film, or web which can be wound into a roll. The extrusion die takes a stream of flowable material and spreads it internally to form a discharging ribbon with a large width-to-thickness ratio.
Dies can also be used to coat. Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, by a layer of fluid. Sometimes, multiple layers of a coating are applied on top of each other. Coatings may also be applied to substrates other than continuous webs such as discrete parts or sheets cut from a web which are passed singularly or in arrays through the coating station. After the deposition of a coating, it can remain a fluid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile fluid to leave behind a solid coat such as a paint, or can be cured or in some other way solidified to a functional coating such as a release coating to which a pressure sensitive adhesive will not aggressively stick.
It is known to provide dies with various manual mechanical, thermomechanical, piezomechanical, magnetostrictive, or motor driven actuators for moving the die lips to provide control of the film, sheet, or coating thickness. This control is produced by controlling the local flow rate of fluid exiting across the width of the die from the slot exit of the discharge slot at the external surface of the die body by adjusting the exit gap. It is known, as shown in FIG. 1, to use a plurality of spaced actuators (not shown) along the movable part of the top die lip across the die width. Each actuator can be adjusted individually to apply an individual displacement force (F.sub.1, F.sub.2, F.sub.3, . . . , F.sub.n) locally to the top lip relative to the opposing fixed bottom lip to control the die slot exit opening. Since at any point across the die width the local discharge rate from the die exit depends upon the local gap, among other factors, the uniformity of flow discharge from the die exit slot can be controlled across the width.
Usually, control is accomplished by measuring the thickness of the film or coating at various points across its width with a thickness gage such as beta-ray, X-ray, or light absorption gage. With the information from such a measurement, an operator can manually adjust a bolt type actuator bearing against the lip. Alternatively, a control system can signal the activation of actuators which bear against the lip or which rotate bolts that bear against the lip. The manual adjustment of die lip flexing bolts by an operator requires skill and experience. It has been shown that the quality of product extruded or coated can be improved by a closed loop control system to replace the manual operator adjustment.
U.S. Pat. No. 2,938,231 discloses thermally expanding and contracting die bolts to slide the lips of a thermal plastic extrusion die to control the slot exit size. The thermal bolt actuators are directly coupled to the die lips. U.S. Pat. No. 3,940,221 discloses an improvement which is used in plastic film extrusion. It provides for both heating and forced cooling on the die lip flexing bolts in conjunction with a continuous film caliper system to control the extruded film thickness across the width of the film. The die slot exit control is directly controlled by bolts that are directly attached to the lip or bear against it.
U.S. Pat. No. 3,122,784 teaches using a plurality of controlled adjustment motors mechanically coupled to lip displacement bolts on a web extrusion die. The motors respond to signals generated by a traversing thickness sensor, and control signals are generated to drive the motors. U.S. Pat. No. 3,819,775 discloses a method in which the die lips are divided into a large number of thermally isolated control zones. The extruded polymer film thickness profile is controlled by selectively controlling the die lip section temperatures. The die lips form the walls of the slot exit so that their temperature influences the local flow properties of the film and the local flow rate exiting the die slot.
U.S. Pat. No. 4,514,348 discloses a combined means for adjusting the die slot exit with a plurality of bolts attached to the die lip across the die width using motor driven bolt rotations for coarse adjustment and thermal expansion or contraction of the bolts fine adjustment.
U.S. Pat. No. 4,594,063 teaches using piezoelectric or magnetostrictive elements bearing against a flexible die lip to adjust the slot exit and control an extruded film.
U.S. Pat. No. 4,871,493 teaches using die lip temperature profiling by lip heaters controlled by the signals from both the extruded thermoplastic temperature and caliper profiles across the die width.
In these known systems, the die slot exit size, also called a gap, is changed by acting directly on the die lips with an attached actuator or by influencing the flow between the die lips by changing the lip temperature. These systems are effectively used for caliper control in the extrusion of single layers. They are also successfully used to control the cross web caliper of the coating applied to substrates. In these situations, only one die exit is present. However, when simultaneous multilayer die coating or simultaneous multilayer extrusion is desired, multiple die lip pairs and multiple die slot exits need to be controlled. Here, the mechanical attachment of actuators or heaters becomes difficult to practically accomplish when three or more coextrusions or simultaneous coatings are desired. Designing and constructing dies so that attachments can be made to at least one lip of the slot exits of three or more exits is impractical.
The use internal choker bars for caliper profile adjustment across the width of a die is known in thermoplastic extrusion, and the use of internal wedges and vanes is described in U.S. Pat. No. 3,464,087 and U.S. Pat. No. 4,533,308. While these mechanisms are useable on multilayer extrusion dies, they are mechanically complex, and use moveable members that extend into high pressure fluid regions within the die body, and present sealing and leakage problems. Other ways of adjusting the cross width flow profile of multi-slot dies are desired.
With simultaneous multilayer slide curtain coating dies as exemplified by U.S. Pat. No. 3,508,947 and as used by the photographic film manufacturing industry, the preferred stacked slot geometry makes it difficult to build moving actuators attached to the lips that change the slot exit dimensions. No such designs are known. Currently, the uniformity of the coated layers depends on the uniformity and precision of making and assembling these dies. This often requires time-consuming ultra-precise, labor intensive final machining by highly skilled personnel. Another practical consideration is that many single layer coating die installations have no means for adjusting the cross web coating profile on-line in response to a control system. A die modification to allow on-line control that can easily be adapted to existing dies is desirable.
In the assembly of single layer dies, it is known that differential torquing of the die plate clamping bolts may affect the uniformity of the die slot exit. The rotation of die bolts to adjust bolt tension levels is not very controllable. High normal forces at threaded interfaces commonly cause stickslip rotational movements of the bolts. This makes continuously variable and small adjustments to bolt tension difficult to accomplish reliably. This technique has not been successfully used for adjustment of coating profiles during operation.